add var_names to to_dataframe

[ahartikainen]

Fixes #2260

Description

Add var_names and var_filter to to_dataframe function. Follows similar logic as in extract function.

Checklist

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2277.org.readthedocs.build/en/2277/

[ahartikainen]

@OriolAbril how should we handle the cases when some variable is not found in all groups. I currently have it in "ignore" mode.

[OriolAbril]

I think that is fine. If I say var_names = ["theta", "diverging", "obs"] I want all their ocurrences to be added to the dataframe, not to error out because diverging is not in the posterior or things like that.

[ahartikainen]

I need to tests this still manually, but I think after it we can merge this.

[codecov]

Codecov Report

Merging #2277 (8b4ef78) into main (eb88f3a) will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main    #2277      +/-   ##
==========================================
- Coverage   86.89%   86.88%   -0.01%     
==========================================
  Files         122      122              
  Lines       12628    12638      +10     
==========================================
+ Hits        10973    10981       +8     
- Misses       1655     1657       +2     

Files	Coverage Δ
arviz/data/inference_data.py	82.56% <91.66%> (+0.04%)	⬆️
arviz/utils.py	88.02% <83.33%> (-0.25%)	⬇️